In general, in a radio communication network in which a plurality of mobile communication terminals participate, when communication is to be directly established between terminals, the terminals must know whether the communication between the terminals which desire communication therebetween with some method can be established or not. This is because a direct wave may not reach the terminals due to some obstacle between the terminals when the terminals are located at such positions that the obstacle exists therebetween.
The terminals seize connection states between the terminals which change with time, i.e., a topology map. When the terminals cannot directly establish communicate with each other, the terminals try to continue the communication by using a method which causes another terminal or the like to relay a radio wave.
As shown in FIG. 17, this topology map describes communication states between the terminals constituting a radio communication network therein. In bi-directional recording of a communication state between terminals, when the number of terminals is represented by n, elements the number of which is n^2−n are necessary.
In FIG. 17, OK indicates that transmission and reception between the corresponding terminals can be performed, and NG indicates that the transmission or reception or both the transmission and reception between the corresponding terminals cannot be performed.
In order to manage the topology maps, at least one management station for managing the topology maps is arranged, and each terminal obtains the topology map from the management station. As another method, the management station of the topology map periodically broadcasts the topology map, and each terminal receives the broadcasted topology map.
As still another method, Japanese Patent Application Laid-Open No. 9-186690 proposes a radio communication system in which identification codes can be transmitted and received between communication apparatuses without arranging a special management apparatus for managing the identification codes of a large number of radio communication apparatuses.
In this radio communication system, one terminal broadcasts an identification code list in which its own code and the identification codes of all the terminals stored by the corresponding terminal are described, and a terminal which receives an identification code list in which its own identification code is not described broadcasts an identification code list in which its identification code and the identification codes of all the other terminals stored in the corresponding terminal.
After a terminal which receives an identification code list which is not stored in the corresponding terminal stores the identification codes of the identification code list, the terminal broadcasts an identification code list in which its own identification code and the identification codes of all the other terminals stored in the corresponding terminal. In this manner, terminals can transmit and receive identification to/from each other. This operation is repeated every predetermined period of time, so that the latest identification code list is used as a topology map.
However, in the method of obtaining a topology map from a topology map management station or causing a topology map management station to broadcast a topology map, when the number of terminals which are accommodated in the same network, the size of the topology map exponentially increases.
Accordingly, traffic required managing the topology map also exponentially increase. For this reason, each time the topology map is transmitted, a communication bandwidth which can be used in communication may be compressed disadvantageously.
For example, in a system in which the number of accommodated terminals is 4, a communication state between terminals is represented by 1 byte, it is assumed that the size of a topology map representing bi-directional communication states between these terminals 12-byte (=4^2−4). In this case, the number of accommodated terminals is doubled, i.e., 8, the size of the topology map is 56-byte (=8^2−8).
When the number of accommodated terminals is trebled, i.e., 12, the size of the topology map is 132-byte (=12^2−12) Similarly, as the number of accommodated terminals increases in almost proportion to the square of a ratio of the numbers of accommodated terminals.
In addition, the system described in Japanese Patent Application Laid-Open No. 9-186690 uses a so-called contention type access method. The contention type access method includes various methods based on CSMA (Carrier Sense Multiple Access) These methods are made on the basis of the following fact. That is, a terminal monitors a communication channel for a predetermined period of time, and uses the channel when the communication channel is not used by other terminals.
For this reason, when a plurality of terminals try to use a communication channel at the same time, a terminal which can use the communication channel cannot easily specified. When the terminals use the communication terminal at almost the same time, the terminals collide with each other in communication, and the communication contents may be damaged.
In order to avoid this collision, a method of selecting a period of time from time when the channel is idle and time when communication is started in the channel at random is generally used. However, in such a case, a time until the communication is started is wasted, and communication start time cannot be easily specified.
As described above, in the method described in Japanese Patent Application Laid-Open No. 9-186690, a time until each terminal transmits an identification code list is not determined, and a problem that a time until the identification code list is propagated to all the terminals cannot be specified is posed.
As the number of stations constituting a network increases, identification code lists easily collide with each other in transmission of the identification code lists. As a result of the collision, a code list disappears, and a correct topology map cannot be obtained. In addition, when the number of terminals described in the identification code list increases, the identification code list is elongated accordingly. A time required to change identification code lists is elongated disadvantageously.
The present invention has been made in consideration of the above problems, and has as its object to provide a radio communication apparatus and a radio communication method which can reduce a communication bandwidth required to manage topology maps and which can control a transmission/reception timing of information for managing the topology maps to prevent the information from being broken by collision of information.